To provide a semiconductor device having electric properties which are not degraded over a long period of time even in a severe environment involving the repeated application of a plurality of thermal shocks, Japanese Patent Laid-Open No. H7(1995)-221235 discloses a structure in which a metal plate having a copper/iron-alloy/copper three-layer structure is interposed between a case electrode and a semiconductor chip. For the same purpose, Japanese Patent Laid-Open No. H4(1992)-229639 proposes a structure in which a semiconductor chip portion is sealed with an epoxy-based insulating member. The resin sealed structure aims at exerting a pressure in a direction perpendicular to the junction surface of the semiconductor chip on a case electrode by utilizing the shrinkage of the resin after molding. On the other hand, Japanese Patent Laid-Open No. H5(1993)-191956 discloses an embodiment which reduces a mechanical stress applied to a semiconductor chip to prevent a chip crack and reduce strain, while Japanese Patent Laid-Open No. H4(1992)-229639 discloses an embodiment which suppresses an increase in electric resistance and a reduction in heat value by retaining a sufficient current carrying capacity and thereby suppresses an abnormal increase in the temperature of a semiconductor chip.
[Patent Document 1]
Japanese Patent Laid-Open No. H7(1995)-221235
[Patent Document 2]
Japanese Patent Laid-Open No. H4(1992)-229639
[Patent Document 3]
Japanese Patent Laid-Open No. H5(1993)-191956
However, the present inventors have found that it is difficult for the foregoing prior art technologies to provide a lead electrode disk with a sufficient current carrying capacity and reduce large thermal strain occurring at the end portion of the bonding member due to the difference in linear expansion coefficient between the lead electrode and the semiconductor chip.
Since the semiconductor device is mounted in the engine room of an automobile, high heat and variations in electric load on a vehicle exert extremely great influences including an increase in the heat value of a generator. In particular, the automobile is located in a severe environment where it experiences repeated cooling and heating over a wide temperature range resulting from a temperature difference between winter and summer or the like so that a semiconductor device with excellent heat dissipation and resistance to thermal fatigue is used preferably.
If a semiconductor device undergoes a plurality of repeated thermal shocks, strain resulting from the difference in linear expansion coefficient between technologies for constructing the semiconductor device is applied to a bonding member such as a solder, which may cause a crack in the bonding member. If a crack occurs, the cross-sectional area of the bonding member as a current carrying path is reduced and an electric resistance is increased so that heat generation is increased, while an amount of heat dissipation through the bonding member is reduced and the temperature of a semiconductor chip is abnormally increased. As a result, the bonding member is molten and the semiconductor chip reaches the heat resistance limit so that a rectifying function disappears. In some cases, the semiconductor device is brought into a breakdown state.
In a structure in which a semiconductor chip is bonded to an element having a linear expansion coefficient greatly different from that of the semiconductor chip by using a bonding member such as a solder, the above-mentioned strain is applied to the bonding member such as a solder on the both surfaces of the semiconductor chip so that it is more difficult to take anti-break down measures for such a structure than for a structure in which a semiconductor chip is wire-bonded.
For example, the embodiment disclosed in Japanese Patent Laid-Open No. H5(1993)-191956 is disadvantageous in that the addition of an intermediate member increases heat resistance to cause temperature elevation and the increased number of components and degraded assemblability lead to an increase in cost. On the other hand, the resin sealed structure disclosed in Japanese Patent Laid-Open No. H7(1995)-221235 or No. H4 (1992)-229639 uses the shrinkage of the resin after molding merely to exert the pressure on the case electrode. Therefore, it is not effective enough to reduce the strain in the bonding member resulting from the difference in linear expansion coefficient and the problem of the strain occurring in the bonding member has not been solved, though the effect of elongating the lifetime has been achieved slightly.
When a lead electrode and a semiconductor chip are connected to each other by using a bonding member such as a solder in such a conventional semiconductor device, a solder layer brought into a molten state by heating is affected by the weight of an object placed on the solder layer and by the surface tension of the molten solder so that variations occur in the thickness of the solder layer. If the solder layer is thinned, the localization of strain to the bonding member resulting from the difference in linear expansion coefficient between the lead electrode and the semiconductor chip is aggravated.